During drilling operations for extraction of hydrocarbons, a variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during drilling. The use of measurement-while-drilling (MWD) and logging-while-drilling (LWD), with real time data transmission, provides substantial benefits during a drilling operation. For example, monitoring of downhole conditions (e.g., temperature, pressure, resistivity, density, and electromagnetic fields) allows for an immediate response to potential well control problems and improves mud programs.
Mud-pulse and electromagnetic telemetries are most commonly used for transmitting downhole data to the surface with a typical 3-10 bits/sec data rate. Acoustic telemetry may provide higher transmission capabilities at 40-80 bits/sec data rates with drill pipe as a transmission line.
While acoustic telemetry may provide fast data rate benefits not possible in mud-pulse and electromagnetic telemetrics, the existing acoustic telemetry technique suffers from signal reflection or transmission loss at each acoustic impedance mismatched interface because existing drillpipe structures lead to formation of frequency stopbands and passbands. In particular, when transmitting acoustic signals within one of the frequency passbands, high data error and low signal-to-noise ratio may result in the loss of the acoustic signals or in the limited transmission range. The frequency stopbands and passbands may drift by thermal induced variations of the pipe length and surrounding acoustic impedance variation, such as the varied mud density. This may limit available acoustic transmission channels and induce signal transmission reliability issues.